Optical fiber and optical fiber preform

Abstract

An optical fiber is constituted by: a three-layer structured core which includes, a first core (having a relative refractive index difference of Δ1 in a region of a radius of R1), a second core (having a relative refractive index difference of Δ2 in a region from the radius R1 to a radius of R2), and a third core (having a relative refractive index difference of Δ3 in a region from the radius of R2 to a radius of R3), wherein the relative refractive index differences have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, thereby satisfying the G652 standard and having an sbs threshold equal to or higher than that of an optical fiber having the same MFD by +3 dB or higher.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is a National Stage of International Application No. PCT/JP2007/067830 filed Sep. 13, 2007, claiming priority based on Japanese Patent Application No. 2006-249360, filed Sep. 14, 2006, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an optical fiber and an optical fiber preform. There was a problem in that when an optical analog signal or an optical baseband signal is transmitted for a long distance by using an optical fiber, due to an effect of stimulated Brillouin scattering (hereinafter, referred to as SBS), only a predetermined intensity of light (SBS threshold power) can be transmitted through the optical fiber although light with higher power was intended to be transmitted, and the remaining light becomes backscattered light and returns to the incoming side, so that transmissible power of signal light is limited. The invention relates to an optical fiber capable of suppressing occurrences of SBS and transmitting higher power of signal light.

BACKGROUND ART

Recently, fiber to the home (FTTH) services in which homes are connected via optical fibers to exchange information using the optical fibers have been expanded. In FTTH services for transmitting various types of information, there is a system for simultaneously transmitting a broadcasting signal and other communications signals in their respective modes using a single optical fiber. In general, in this system, in many cases, the broadcasting signal is an analog signal, a baseband signal, or an optical SCM signal. The characteristics of this system from the point of optical fiber as a transmission medium are as follows:

(i) FTTH is based on a typical double-star PON (passive optical network), and a distribution loss increases.

(ii) In order to transmit an analog signal, a baseband signal, or an optical SCM signal, a receiver needs a high CNR (carrier-to-noise ratio), so that the minimum signal light power of a light receiving unit needs to be greater than that in digital transmission used for communications.

As described above, when analog transmission using intensity modulation is performed for video transmission, high power transmission is needed to compensate for a distribution loss and guarantee a high CNR. However, there was a problem in that only a predetermined intensity of light (SBS threshold power) can be transmitted through the optical fiber although higher power is intended to be transmitted, and the remaining light becomes backscattered light and returns to the incident side, so that transmissible signal light power was limited.

As means for suppressing SBS, there is a technique for changing a dopant concentration and a residual stress in a longitudinal direction (for example, see Patent Document 1). In this technique, by changing the dopant concentration or the residual stress in the longitudinal direction, it is possible to enlarge the Brillouin spectrum and suppress occurrences of SBS. In addition, techniques for giving such a refractive index profile for an optical fiber that SBS can be suppressed, are proposed (for example, see Patent Documents 2 to 5, and 7).

Patent Document 1: Japanese Patent No. 2584151

Patent Document 2: WO 2004/1100406

Patent Document 3: U.S. Pat. No. 7,082,243

Patent Document 4: Japanese Unexamined Patent Application Publication No. 2006-154707

Patent Document 5: Japanese Unexamined Patent Application Publication No. 2006-184534

Patent Document 6: Japanese Unexamined Patent Application Publication No. 2006-133314

Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-154713

Non-Patent Document 1: “Design concept for optical fibers with enhanced SBS threshold” Optics Express, Vol. 13, Issue 14, p. 5338 (July 2005), Andrey

Non-Patent Document 2: “Nonlinear Optical Fibers with Increased SBS Thresholds” OFCINFOEC 2006, OTuA3, Scott Bickham, Andrey Kobyakov, Shenping Li.

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

As a technique for suppressing SBS, as described above, the technique for changing a dopant concentration or a residual stress in a longitudinal direction is reported (Patent Document 1). However, in this technique, optical characteristics are changed in the longitudinal, so that this technique is not preferable for practical use.

In addition, techniques for suppressing SBS by giving a refractive index profile for an optical fiber (Patent Documents 2 to 5 and 7) are reported. In theses techniques, a change in optical characteristics in a longitudinal direction does not occur. However, a structure having a refractive index profile suitable for desired characteristics is needed.

In Patent Documents 2, 3, and 5, for an optical fiber described which has a three-layer structured refractive index profile, the refractive index profile is set to a suitable condition, thereby suppressing SBS and obtaining the same optical characteristics as ITU-T Recommendation G.652 hereinafter, referred to as G652). However, not all structures described in Patent Documents 2, 3, and 5 satisfy the same optical characteristics as G652, and when manufacturing is practically performed on the basis of these conditions, a suitable design value for each condition is needed.

In Patent Document 4, a uniform bending loss becomes worse, and in consideration of handling of the optical fiber, this phenomenon is not preferable.

In Patent Document 6, fluorine needs to be added at an intentional position, so that manufacturing a preform using a VAD method is difficult.

In Patent Document 7, only the shape of a refractive index profile is described, and detailed parameters and the like are not mentioned.

Means for Solving the Problems

The invention is designed to solve the above-mentioned problems. An object of the invention is to provide an optical fiber and an optical fiber preform which have stable characteristics in a longitudinal direction, compatibility with G652, excellent manufacturability, and suppressed SBS, by giving a structure design value for a suitable refractive index profile.

According to a first aspect of the invention, there is provided an optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive, relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

According to a second aspect of the invention, there is provided an optical fiber having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, the Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 nm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

According to a third aspect of the invention, there is provided an optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, the Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, and when the optic preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

According to a fourth aspect of the invention, there is provided an optical fiber preform having a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, a when the optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a relationship between incoming light power and transmitted, backscattered light power to show occurrences of SBS.

FIG. 2 is a view illustrating a configuration of an SBS threshold measurement system.

FIG. 3 is a view showing MUD dependence of an SBS threshold of an SMF having a refractive index profile of step-index model illustrated in FIG. 4.

FIG. 4 is a view showing the refractive index profile of step-index model,

FIG. 5 is a view showing a first example of a refractive index profile of an optical fiber related to the invention.

FIG. 6 is a view showing a second example of a refractive index profile of an optical fiber related to the invention.

FIG. 7 is a view showing a relationship between X+Y and an SBS suppression effect (SBSeff) on an SMF having the same MFD when Δ1−Δ2=X AND Δ3−Δ2=Y.

FIG. 8 is a view showing a relationship of (X,Y) satisfying a zero-dispersion wavelength of 1300 to 1324 nm and SBSeff≧+3 dB.

FIG. 9 is a view showing a relationship between Δ2+Δ3 and R2/R1 satisfying MFD=7.9 to 10.2 μm.

FIG. 10 is a view showing a relationship between Δ3−Δ1 and SBSeff.

FIG. 11 is a view showing a relationship between Δ1−Δ2 and SBSeff.

FIG. 12 is a view showing a relationship between Δ1−Δ2 and SBSeff.

FIG. 13 is a view showing relationships between SBS thresholds and MFDs of Examples 1 and 1a to 1g.

FIG. 14 is a view showing relationships between SBS thresholds and MFDs of Examples 1 and 1a to 1v.

FIG. 15 is a view showing relationships between SBS thresholds and MFDs of Examples 2a to 2f.

FIG. 16 is a view showing relationships between SBS thresholds and MFDs of Examples 2g to 2m.

FIG. 17 is a view showing a refractive index profile of an optical fiber of Example 3.

FIG. 18 is a view showing a refractive index profile of an optical fiber preform of Example 5.

FIG. 19 is a view showing relationships between SBS thresholds and MFDs of Example 5.

FIG. 20 is a view showing a refractive index profile of an optical fiber preform of Example 6.

FIG. 21 is a view showing relationships between SBS thresholds and MFDs of Example 6.

FIG. 22 is a view showing a refractive index profile of an optical fiber preform of Example 7.

DESCRIPTION OF SYMBOLS

• • 1: LIGHT SOURCE AT WAVELENGTH OF 1.32 μm
  • 2: LIGHT SOURCE AT WAVELENGTH OF 1.55 μm
  • 3: EDFA
  • 4: POWER METER FOR MEASURING BACKSCATTERED LIGHT POWER
  • 5: 9:1 COUPLER
  • 6: POWER METER FOR MEASURING INCOMING LIGHT POWER
  • 7: POWER METER FOR MEASURING TRANSMITTED LIGHT POWER
  • 8: OPTICAL FIBER TO BE MEASURED

BEST MODE FOR CARRYING OUT THE INVENTION

An optical fiber of the invention has a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, or has a refractive index profile constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index.

In the optical fiber, Δ2 is equal to or less than 0.4%, Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1, when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%, Δ2, Δ3, R1, an R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform beading loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold at a wavelength of 1.55 μM is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

In addition, the optical fiber of the invention may be manufactured by drawing a preform manufactured by a typical VAD method. However, the manufacturing method of the preform is not limited to the VAD method, and well-known OVD and CVD methods can be used.

FIG. 1 is a graph showing occurrences of SBS in an optical fiber. As shown in FIG. 1, when the power of light entering the optical fiber is gradually increased, at a specific value, the power of backscattered light sharply increases, and most of the power of the incoming light becomes backscattered light and returns to the incoming side. Therefore, as shown in FIG. 1, incoming light power at the maximum change rate (second-order differential equation of backscattered light) of a gradient of the backscattered light with respect to a change in the incoming light power is defined as a threshold (SBS threshold) of an occurrence of SBS.

FIG. 2 is a view illustrating a configuration of an SBS threshold measurement system. In FIG. 2, reference numeral 1 denotes a light source at a wavelength of 1.32 μm, reference numeral 2 denotes a light source at a wavelength of 1.55 μm, reference numeral 3 denotes an EDFA, reference numeral 4 denotes a power meter for measuring backscattered light power, reference numeral 5 denotes a 9:1 coupler, reference numeral 6 denotes a power meter for measuring incoming light power, reference numeral 7 denotes a power meter for measuring transmitted light power, and a reference numeral 8 denotes an optical fiber to be measured. In this measurement system, the three power meters 4, 6, and 7 are connected through the 9:1 coupler 5 to measure the power of incoming light, backscattered light, and transmitted light of the optical fiber 8 to be measured. In addition, incoming light power at the maximum second-order differential equation of the backscattered light with respect to the incoming light becomes an SBS threshold.

In Patent Documents 2 and 3, an SBS threshold is evaluated by using the same measurement system and definition. Here, the definition of the SBS threshold is considered as definition 4 in the following document.

Shimizu, “A Study on Definition of the SBS Threshold in a Single-Mode Optical Fiber”, Institute of Electronics, Information and Communication Engineers, General Conference in 2005, B-10-66.

The SBS threshold is dependent on a mode field diameter (hereinafter, referred to as MFD). A plot result obtained by calculating the MFD dependence of the SBS threshold of a single-mode optical fiber (hereinafter, referred to as SMF) which has a typical refractive index profile of step-index model as shown in FIG. 4 and satisfies the G652 standard is shown in FIG. 3. As shown in FIG. 3, in the range of 7.9 to 10.2 μm of an MFD at a wavelength of 1.31 μm that is in the G652 standard, the SBS threshold of the SMF changes in the range of 7.4 to 9.7 dBm. Accordingly, when the SBS threshold is compared, an optical fiber having the same MFD needs to be compared.

The optical fiber of the invention has optical characteristics described in G652 in that a cable cutoff wavelength is less than 1260 nm, an MFD at a wavelength of 1.31 μm is in the range of 7.9 to 10.2 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a bending diameter of 20 mm and a wavelength of 1.31 μm, and an SBS threshold is equal to or higher than two times (+3 dB) that of an SMF having the same MFD.

FIG. 5 is a view illustrating a first example of a refractive index profile of the optical fiber of the invention. The refractive index profile is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ1>Δ2 and Δ3>Δ2.

The refractive index profile of the optical fiber of the invention, as illustrated in FIG. 6, may allow relative refractive index differences not to have uniform values. FIG. 6 is a view illustrating a second example of a refractive index profile of the optical fiber of the invention. The refractive index profile is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index, wherein Δ1>Δ2 and Δ3>Δ2.

For the optical fiber having the refractive index profile constituted by the three-layer structured core and the one-layer structured cladding for surrounding the core, in order to obtain an optical fiber which has the above-mentioned optical characteristics, that is, characteristics compatible with G652 and an SBS threshold equal to or higher than two times that of (by +3 db) the SMF having the same MFD as the optical fiber of the invention, detailed examinations were repeated. As a result, it was found that relationships between Δ1, Δ2, Δ3, R1, R2, and R3 were limited.

FIG. 7 is a view showing a relationship of a SBS suppression effect SBSeff on an SMF when Δ1−Δ2=X and Δ3−Δ2=Y. Here, SBSeff is defined by the following expression.
SBSeff=SBS threshold of the optical fiber of the invention−SBS threshold of the SMF having the same MFD as the optical fiber of the invention

As can be seen from FIG. 7, by allowing X+Y to be greater than 0.4%, it is possible to improve SBSeff to be equal to or higher than +3 dB and increase the SBS threshold. However, by only using the condition, in some cases, an optical fiber having the optical characteristics compatible with the G652 cannot be obtained.

Specifically, in order to obtain an optical fiber having a zero-dispersion wavelength in the range of 1300 to 1324 nm, it is preferable that X and Y have relationships of X<0.6%, 0.1%≦Y≦0.6%, and (2*X−0.7)%<Y<(X/2+0.4)%. The relationships of X and Y satisfying this condition are shown in FIG. 8.

In addition, in order to allow the MFD at the wavelength of 1.31 μm to be 7.9 μm, that is, the lower limit of the G652 standard, or greater, while the condition is satisfied, it is preferable that Δ2 be equal to or less than 0.4%. In addition, in order to allow the MFD at the wavelength of 1.31 μm to be in the range of 7.9 to 10.2 μm, the position of the third core in the radial direction represented as R2/R1 needs to be suitably determined depending on the sum of Δ2 and Δ3, that is, Δ2+Δ3.

In order to obtain an optical fiber for enabling the MFD at the wavelength of 1.31 μm to be in the range of 7.9 to 10.2 μm, it is preferable that (Δ2+Δ3) and R2/R1 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15. The relationships between (Δ2+Δ3) and R2/R1 satisfying the condition are shown in FIG. 9.

In addition, from the point of manufacturability of an optical fiber, it is preferable that Δ3≧Δ1.

FIG. 10 is a view showing a relationship between Δ3−Δ1 and SBSeff. As shown in FIG. 10, when Δ3−Δ1 is negative, SBSeff significantly changes with a slight change in relative refractive index difference. On the other hand, when Δ3−Δ1 is positive, the change rate of SBSeff with respect to the change in relative refractive index difference is small. In addition, when SBSeff is approximated to a quadratic equation of Δ3−Δ1, the approximation curve is a parabola which opens down, and the inflection point is Δ3−Δ1>0. It can be seen that the effect of SBSeff caused by the change in relative refractive index difference is small when Δ3−Δ1 is positive.

When an optical fiber preform is manufactured, in some cases, due to fluctuation in dopant concentration, the relative refractive index difference is changed from a target by about ±0.05%. In this case, there is a possibility that SBSeff is decreased to be lower than a target. When Δ3−Δ1 is positive, the change rate of SBSeff caused by the change in relative refractive index difference is small. Therefore, SBSeff does not significantly change with respect to the change in relative refractive index difference caused by the fluctuation in dopant concentration, and SBS characteristics that are always stable can be obtained.

In addition, in the refractive index profile of FIG. 10, Δ1 is 0.5%, Δ2 is 0.22%, Δ3 is in the range of 0.40 to 0.65% at an interval of 0.025%, and R2/R1 is 2.2. However, as shown in Table 1, in other combinations of Δ1, Δ2, Δ3, R1, R2, and R3, the inflection point is Δ3−Δ1>0, and it can be seen that SBS characteristics which are always stable can be obtained when Δ3−Δ1 is positive.

It is preferable that Δ1−Δ2 be equal to or greater than 0.25%. FIG. 11 is a view showing a relationship between Δ1−Δ2 and SBSeff. As shown in FIG. 11, when Δ1 is greater than Δ2 by 0.25% or higher, SBSeff can be significantly increased as compared with the case where the difference between Δ1 and Δ2 is equal to or smaller than 0.25%, thereby obtaining a higher SBS suppression effect. In addition, when Δ1−Δ2 is equal to or greater than 0.25%, the effect of the change in relative refractive index difference caused by fluctuation in dopant concentration during manufacturing of the optical fiber preform is reduced. Therefore, even when Δ1−Δ2 is changed, SBSeff does not change significantly, and degradation in yield can be prevented.

In the refractive index profile of FIG. 11, Δ1 is in the range of 0.44% to 0.56% at an interval of 0.03%, Δ2 is 0.24%, Δ3 is 0.55%, and R2/R1 is 2.2. However, as shown in FIG. 12, in a refractive index profile using other combinations of Δ1, Δ2, Δ3, R1, R2, and R3, that is, when Δ1 is in the range of 0.44% to 0.56%, Δ2 is in the range of 0.18% to 0.26%, Δ3 is in the range of 0.45% to 0.65%, and R2/R1 is in the range of 1.8 to 2.6, the same tendency can be obtained. Particularly, in a refractive index profile in which Δ1−Δ2 is equal to or greater than 0.25%, a higher SBS suppression effect can be obtained, and dependence of SBSeff on a change in Δ1−Δ2 is reduced.

	TABLE 1
	Δ1	Δ2	R2/R1	Inflection point (Δ3 − Δ1)
	%	%	—	%
	0.5	0.22	2.2	0.10
	0.5	0.18	2.2	0.18
	0.5	0.26	2.2	0.02
	0.44	0.22	2.2	0.02
	0.56	0.22	2.2	0.12
	0.5	0.22	1.6	0.05
	0.5	0.22	2	0.10
	0.5	0.22	2.4	0.01

In addition, by disposing the third core as described above, it is possible to obtain the SBS threshold higher than that of a conventional optical fiber by +3 dB or higher and obtain characteristics compatible with the G652.

EXAMPLES

Example 1 and Comparative Example 1

In Table 2, structural parameters and optical characteristics of an optical fiber of Example 1 which has the refractive index profile of FIG. 5 are shown. In addition, structural parameters and optical characteristics of an optical fiber of Comparative Example 1 are shown. The optical fiber of Comparative Example 1 is an SMF having a step-index profile as illustrated in FIG. 4.

	TABLE 2
	Unit	Example 1	Comparative 1
R1	μm	1.35	4.39
R2	μm	2.97	—
R3	μm	4.32	—
R2/R1	—	2.2	—
Δ1	%	0.50	0.35
Δ2	%	0.20	—
Δ3	%	0.50	—
Δ1 − Δ2	%	0.30	—
Δ3 − Δ2	%	0.30	—
Δ3 + Δ2	%	0.70	—
Fiber cutoff wavelength	μm	1.32	1.28
Cable cutoff wavelength	μm	1.24	1.23
MFD(Petermann II)	μm	9.2	9.2
at 1.31 μm
Zero-dispersion	nm	1321.1	1309.2
wavelength
Zero-dispersion slope	ps/nm2-km	0.0888	0.0882
20 mm diameter	dB/m	0.08	0.11
bending loss
at 1.31 μm
SBS threshold	dBm	12.2	8.7
at 1.55 μm 20 km

As shown in Table 2, the optical fiber having the structural parameters of Example 1 related to the invention had an SBS threshold of 12.2 dBm for a length of 20 km, and could obtain a higher suppression effect than the optical fiber of Comparative Example 1 which had the same MFD by +3.5 dB. In addition, the optical fiber of Example 1 had the same optical characteristics as the SMF of Comparative Example 1 and satisfied the G652 standard.

Examples 1a to 1g

Table 3 shows results in the case where structural parameters of Example 1 are represented by using X, Y, and R2/R1 described above.

	TABLE 3
	Unit	Example 1a	Example 1b	Example 1c	Example 1d	Example 1e	Example 1f	Example 1g
R1	μm	1.53	1.50	1.35	1.26	1.16	1.03	1.03
R2	μm	2.91	3.29	2.98	3.15	2.91	3.21	3.20
R3	μm	4.44	4.79	4.34	4.41	4.08	4.24	4.24
R2/R1	—	1.9	2.2	2.2	2.5	2.5	3.1	3.1
Δ1	%	0.50	0.50	0.40	0.60	0.30	0.50	0.60
Δ2	%	0.05	0.20	0.20	0.20	0.20	0.20	0.20
Δ3	%	0.50	0.40	0.50	0.50	0.60	0.60	0.60
Δ1 − Δ2	%	0.45	0.30	0.20	0.40	0.10	0.30	0.40
Δ3 − Δ2	%	0.45	0.20	0.30	0.30	0.40	0.40	0.40
Δ3 + Δ2	%	0.55	0.60	0.70	0.70	0.80	0.80	0.80
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32
Wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.47	9.56	9.48	9.11	9.22	9.45	9.27
at 1.31 μm
Zero-dispersion	nm	1321.6	1314.3	1309	1321.1	1323.7	1314.7	1317.3
wavelength
Zero-dispersion	ps/nm2-km	0.09	0.0907	0.0892	0.0899	0.0895	0.0895	0.0893
slope
20 mm diameter	dB/m	0.39	0.19	0.24	0.07	0.30	0.30	0.16
bending loss
at 1.31 μm
SBS threshold	dBm	13.1	13.1	12.5	13.3	12.4	13.2	12.9
at 1.55 μm 20 km

The optical fibers having the structural parameters of Examples 1a to 1g shown in Table 3 had SBS thresholds of 12.4 to 13.3 dBm for a length of 20 km as shown in FIG. 13 and could obtain suppression effects higher than the SMF having the same MFD by +3.7 to +4.6 dB. In addition, the optical characteristics of the optical fibers of Examples 1a to 1g all satisfied the G652 standard.

Examples 1h to 1v

Tables 4 and 5 show results in the case where structural parameters of Example 1 are represented by using X, Y, and R2/R1 described above. The optical fibers having the structural parameters of Examples 1 a to 1g shown in Table 3 and Examples 1h to 1v shown in Tables 4 and 5 had SBS thresholds of 10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 14 and could obtain suppression effects higher than the SMF having the same MFD by +3.1 to +4.5 dB. In addition, the optical characteristics of the optical fibers of Examples 1h to 1v all satisfied the G652 standard.

	TABLE 4
	Unit	Example 1h	Example 1i	Example 1j	Example 1k	Example 1l	Example 1m	Example 1n
R1	μm	1.81	1.69	3.14	1.33	2.80	1.12	1.74
R2	μm	2.90	3.21	5.81	3.33	5.33	3.48	6.80
R3	μm	4.71	4.90	8.95	4.67	8.13	4.60	8.54
R2/R1	—	1.6	1.9	1.9	2.5	1.9	3.1	3.9
Δ1	%	0.50	0.50	0.50	0.50	0.40	0.50	0.60
Δ2	%	0.10	0.10	0.20	0.10	0.20	0.20	0.20
Δ3	%	0.40	0.40	0.40	0.50	0.50	0.50	0.60
Δ1 − Δ2	%	0.40	0.40	0.30	0.40	0.20	0.30	0.40
Δ3 − Δ2	%	0.30	0.30	0.20	0.40	0.30	0.30	0.40
Δ3 + Δ2	%	0.50	0.50	0.60	0.60	0.70	0.70	0.80
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.23	9.7	9.2	10.07	9.2	9.93	9.8
at 1.31 μm
Zero-dispersion	nm	1321.8	1322.8	1316.5	1317.9	1310	1307.6	1312
wavelength
Zero-dispersion	ps/nm2-km	0.092	0.093	0.092	0.093	0.091	0.092	0.092
slope
20 mm diameter	dB/m	0.08	0.48	0.05	1.91	0.08	0.72	0.70
bending loss
at 1.31 μm
SBS threshold	dBm	12.1	13.4	12.1	13.8	11.8	13.1	13.1
at 1.55 μm 20 km

	TABLE 5
	Unit	Example 1o	Example 1p	Example 1q	Example 1r	Example 1s	Example 1t	Example 1u	Example 1v
R1	μm	1.47	1.65	1.11	2.57	1.04	1.16	3.71	1.15
R2	μm	6.85	6.28	2.78	4.86	2.60	2.90	5.94	7.25
R3	μm	8.32	7.93	3.89	7.44	3.63	4.06	9.65	8.40
R2/R1	—	4.7	3.8	2.5	1.9	2.5	2.5	1.6	6.3
Δ1	%	0.60	0.70	0.40	0.40	0.60	0.60	0.40	0.30
Δ2	%	0.20	0.20	0.30	0.20	0.30	0.20	0.04	0.20
Δ3	%	0.70	0.70	0.60	0.60	0.70	0.60	0.40	0.90
Δ1 − Δ2	%	0.40	0.50	0.10	0.20	0.30	0.40	0.36	0.10
Δ3 − Δ2	%	0.50	0.50	0.30	0.40	0.40	0.40	0.36	0.70
Δ3 + Δ2	%	0.90	0.90	0.90	0.80	1.00	0.80	0.44	1.10
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.8	9.2	8.6	8.6	8.0	8.8	10.2	9.9
at 1.31 μm
Zero-dispersion	nm	1314.4	1322	1314.9	1322.3	1323.8	1321.2	1308.3	1324
wavelength
Zero-dispersion	ps/nm2-km	0.093	0.091	0.089	0.089	0.087	0.090	0.093	0.093
slope
20 mm diameter	dB/m	0.92	0.17	0.01	0.02	0.00	0.03	1.20	1.80
bending loss
at 1.31 μm
SBS threshold	dBm	13.2	13.0	11.4	11.3	10.9	12.2	12.8	13.2
at 1.55 μm 20 km

Examples 2a to 2f

Table 6 shows optical characteristics in the case where structural parameters of a refractive index profile of the optical fiber having the refractive index profile of FIG. 6 are represented by using X, Y, and R2/R1 described above.

	TABLE 6
	Unit	Example 2a	Example 2b	Example 2c	Example 2d	Example 2e	Example 2f
R1	μm	1.51	1.39	1.43	1.31	1.11	1.33
R2	μm	3.03	3.05	2.87	3.13	3.10	3.45
R2	μm	4.60	4.49	4.36	4.49	4.25	4.83
R2/R1	%	2.00	2.20	2.00	2.40	2.80	2.60
Δ1	%	0.50	0.50	0.44	0.56	0.44	0.40
Δ2	%	0.24	0.22	0.18	0.18	0.26	0.30
Δ3	%	0.50	0.60	0.70	0.70	0.70	0.70
Δ1 − Δ2	%	0.26	0.28	0.26	0.38	0.18	0.10
Δ3 − Δ2	%	0.26	0.38	0.52	0.52	0.44	0.40
Δ3 + Δ2	%	0.74	0.82	0.88	0.88	0.96	1.00
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.2	9.36	9.44	9.39	9.37	9.1
at 1.31 μm
Zero-dispersion	nm	1317.9	1314.9	1314.2	1322.6	1309.5	1308.1
wavelength
Zero-dispersion	ps/nm2-km	0.0906	0.0905	0.0902	0.0914	0.0897	0.0898
slope
20 mm diameter	dB/m	0.39	0.69	1.42	1.35	0.92	0.15
bending loss
at 1.31 μm
SBS threshold	dBm	12.0	12.8	13.0	13.7	12.3	12.0
at 1.55 μm

The optical fibers having the structural parameters of Examples 2a to 2f shown in Table 6 had SBS thresholds of 12.0 to 13.7 dBm for a length of 20 km as shown in FIG. 15 and could obtain suppression effects higher than the SMF having the same MFD by +3.3 to +5.0 dB. In addition, the optical characteristics of the optical fibers of Examples 2a to 2f all satisfied the G652 standard.

In addition, by changing the refractive index of the third core as illustrated in FIG. 6, the amount of dopant GeO₂ in the core can be reduced, so that it is possible to reduce a loss in the optical fiber.

Examples 2g to 2m

Table 7 shows optical characteristics in the case where structural parameters of a refractive index profile of the optical fiber having the refractive index profile of FIG. 6 are represented by using X, Y, and R2/R1 described above. The optical fibers having the structural parameters of Examples 2a to 2f shown in Table 6 and Examples 2g to 2m shown in Table 7 had SBS thresholds of 10.8 to 14.3 dBm for a length of 20 km as shown in FIG. 16 and could obtain suppression effects higher than the SMF having the same MFD by +3.2 to +4.7 dB. In addition, the optical characteristics of the optical fibers of Examples 2g to 2m all satisfied the G652 standard.

	TABLE 7
	Unit	Example 2g	Example 2h	Example 2i	Example 2j	Example 2k	Example 2l	Example 2m
R1	μm	1.56	1.51	1.68	1.58	1.81	1.35	1.41
R2	μm	3.13	3.33	3.69	4.10	3.62	2.97	2.82
R3	μm	4.76	4.91	5.43	5.74	5.51	4.37	4.28
R2/R1	—	2.0	2.2	2.2	2.6	2.0	2.2	2.0
Δ1	%	0.56	0.56	0.50	0.50	0.44	0.50	0.44
Δ2	%	0.24	0.22	0.15	0.18	0.18	0.35	0.20
Δ3	%	0.70	0.70	0.70	0.55	0.55	0.80	0.70
Δ1 − Δ2	%	0.32	0.34	0.35	0.32	0.26	0.15	0.24
Δ3 − Δ2	%	0.46	0.48	0.55	0.37	0.37	0.45	0.50
Δ3 + Δ2	%	0.94	0.92	0.85	0.73	0.73	1.15	0.90
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	8.54	8.83	9.64	10.03	9.77	8.1	9.2
at 1.31 μm
Zero-dispersion	nm	1322.4	1321.7	1317.1	1315.6	1310.9	1317	1314
wavelength
Zero-dispersion	ps/nm2-km	0.088	0.090	0.093	0.093	0.092	0.087	0.090
slope
20 mm diameter	dB/m	0.01	0.05	0.40	1.36	0.64	0.00	0.18
bending loss
at 1.31 μm
SBS threshold	dBm	11.7	12.6	13.7	14.3	13.1	10.8	12.6
at 1.55 μm 20 km

Examples 3 and 4

FIG. 17 shows a refractive index profile of an optical fiber of Example 3 related to the invention. The refractive index profile in Example 3, as shown in FIG. 17, is constituted by: a three-layer structured tore which includes, in the central portion of the core, a first core in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core in a region from the radius of R2 μam to a radius of R3 μm; and a cladding which surrounds and comes in contact with the three-layer structured core. However, unlike in Examples 1 and 2, the refractive index profile of the core smoothly changes, and the boundary thereof is obscure. Therefore, by using the change rate (dΔ/dr) of the relative refractive index difference in the radial direction, the diameter of each layer was determined. The relative refractive index difference Δ1 of the first core, as represented by the following equation (1), is defined as Δ which becomes equivalently uniform in the region from the center of the core to the radius R1, the relative refractive index difference Δ2 of the second core is defined as a relative refractive index difference that becomes a minimum value in the region between the radii R1 and R2 μm, and the relative refractive index difference Δ3 of the third core is defined as a relative refractive index difference that becomes a maximum value in the region between the radii R2 and R3 μm.

$\begin{array}{cc}\mathrm{Equation}1& \\ \Delta 1=\frac{{\int }_0^{R1}\Delta \left(r\right)rdr}{{\int }_0^{R1}rdr}& \left(1\right)\end{array}$

The structural parameters of the optical fiber of Example 3 defined as described above and the optical characteristics thereof are shown in Table 8. In addition, in Table 8, the structural parameters of the optical fiber of Example 4 having the same refractive index profile as Example 3 and the optical characteristics thereof are shown.

	TABLE 8
	Unit	Example 3	Example 4
R1	μm	1.36	1.73
R2	μm	3.11	3.16
R3	μm	4.58	4.67
R2/R1	—	2.29	1.83
Δ1	%	0.50	0.53
Δ2	%	0.18	0.18
Δ3	%	0.48	0.53
Δ1 − Δ2	%	0.32	0.35
Δ3 − Δ2	%	0.30	0.35
Δ3 + Δ2	%	0.66	0.71
Fiber cutoff wavelength	μm	1.32	1.32
Cable cutoff wavelength	μm	1.24	1.23
MFD(Petermann II)	μm	9.55	9.25
at 1.31 μm
Zero-dispersion	nm	1316.4	1320.4
wavelength
Zero-dispersion slope	ps/nm2-km	0.089	0.089
20 mm diameter	dB/m	0.23	0.07
bending loss
at 1.31 μm
SBS threshold	dBm	12.7	12.2
at 1.55 μm 20 km
loss at 1.55 μm	dB/km	0.191	0.196

As shown in Table 8, the optical fibers of Examples 3 and 4 had SBS thresholds of 12.2 to 12.7 dBm for a length of 20 km, and could obtain higher suppression effects than the SMF having the same MFD by +3.5 to +4.0 dB. In addition, the optical fibers of Examples 3 and 4 all satisfied the G652 standard.

Example 5

FIG. 18 is a refractive index profile of an optical fiber preform of Example 5. The optical fiber preform in this example, as shown in FIG. 18, is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm. Like Examples 1 and 2, the three-layer structured core is included.

Structural parameters of the optical fiber preform of Example 5 are represented in Tables 9 and 10 by using X, Y, and R2/R1 described above, and optical characteristics that were exhibited when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 5 had an SBS threshold of 10.9 to 13.8 dBm for a length of 20 km as shown in FIG. 19, and obtained a suppression effect higher than the SMF having the same MFD by +3.1 to +4.5 dB, so that the G652 standard was further satisfied.

	TABLE 9
	Unit	Example 5a	Example 5b	Example 5c	Example 5d	Example 5e	Example 5f	Example 5g
R1	μm	1.32	1.81	1.69	3.14	1.33	2.80	1.12
R2	μm	2.75	2.90	3.21	5.81	3.33	5.33	3.48
R3	μm	4.04	4.71	4.90	8.95	4.67	8.13	4.60
R2/R1	—	2.1	1.6	1.9	1.9	2.5	1.9	3.1
Δ1	%	0.50	0.50	0.50	0.50	0.50	0.40	0.50
Δ2	%	0.20	0.10	0.10	0.20	0.10	0.20	0.20
Δ3	%	0.60	0.40	0.40	0.40	0.50	0.50	0.50
Δ1 − Δ2	%	0.30	0.40	0.40	0.30	0.40	0.20	0.30
Δ3 − Δ2	%	0.40	0.30	0.30	0.20	0.40	0.30	0.30
Δ3 + Δ2	%	0.80	0.50	0.50	0.60	0.60	0.70	0.70
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	124	1.24
wavelength
MFD(Petermann II)	μm	8.89	9.23	9.7	9.2	10.07	9.2	9.93
at 1.31 μm
Zero-dispersion	nm	1318.5	1321.8	1322.8	1316.5	1317.9	1310	1307.6
wavelength
Zero-dispersion slope	ps/nm2-km	0.090	0.092	0.093	0.092	0.093	0.091	0.092
20 mm diameter	dB/m	0.03	0.08	0.48	0.05	1.91	0.08	0.72
bending loss
at 1.31 μm
SBS threshold	dBm	12.0	12.1	13.4	12.1	13.8	11.8	13.1
at 1.55 μm 20 km

	TABLE 10
	Unit	Example 5h	Example 5i	Example 5j	Example 5k	Example 5l	Example 5m	Example 5n	Example 5o
R1	μm	1.47	1.65	1.11	2.57	1.04	1.16	3.71	1.15
R2	μm	6.85	6.28	2.78	4.86	2.60	2.90	5.94	7.25
R3	μm	8.32	7.93	3.89	7.44	3.63	4.06	9.65	8.40
R2/R1	—	4.7	3.8	2.5	1.9	2.5	2.5	1.6	6.3
Δ1	%	0.60	0.70	0.40	0.40	0.60	0.60	0.40	0.30
Δ2	%	0.20	0.20	0.30	0.20	0.30	0.20	0.04	0.20
Δ3	%	0.70	0.70	0.60	0.60	0.70	0.60	0.40	0.90
Δ1 − Δ2	%	0.40	0.50	0.10	0.20	0.30	0.40	0.36	0.10
Δ3 − Δ2	%	0.50	0.50	0.30	0.40	0.40	0.40	0.36	0.70
Δ3 + Δ2	%	0.90	0.90	0.90	0.80	1.00	0.80	0.44	1.10
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.8	9.2	8.6	8.6	8.0	8.8	10.2	9.9
at 1.31 μm
Zero-dispersion	nm	1314.4	1322	1314.9	1322.3	1323.8	1321.2	1308.3	1324
wavelength
Zero-dispersion	ps/nm2-km	0.093	0.091	0.089	0.089	0.087	0.090	0.093	0.093
slope
20 mm diameter	dB/m	0.92	0.17	0.01	0.02	0.00	0.03	1.20	1.80
bending loss
at 1.31 μm
SBS threshold	dBm	13.2	13.0	11.4	11.3	10.9	12.2	12.8	13.2
at 1.55 μm 20 km

Example 6

FIG. 20 shows a refractive index profile of an optical fiber preform of Example 6. The optical fiber preform in this example, as shown in FIG. 20, is constituted by: a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm. Like Examples 1 and 2, the three-layer structured core is included.

Structural parameters of the optical fiber preform of Example 6 are represented in Tables 11 and 12 by using X, Y, and R2/R1 described above, and optical characteristics that were exhibited when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 6 had an SBS threshold of 10.8 to 14.3 dBm for a length of 20 km as shown in FIG. 21, and obtained a suppression effect higher than the SMF having the same MFD by +3.2 to +4.7 dB, so that the G652 standard was further satisfied.

	TABLE 11
	Unit	Example 6a	Example 6b	Example 6c	Example 6d	Example 6e	Example 6f
R1	μm	1.51	1.39	1.43	1.31	1.11	1.33
R2	μm	3.03	3.05	2.87	3.13	3.10	3.45
R3	μm	4.60	4.49	4.36	4.49	4.25	4.83
R2/R1	—	2.00	2.20	2.00	2.40	2.80	2.60
Δ1	%	0.50	0.50	0.44	0.56	0.44	0.40
Δ2	%	0.24	0.22	0.18	0.18	0.26	0.30
Δ3	%	0.50	0.60	0.70	0.70	0.70	0.70
Δ1 − Δ2	%	0.26	0.28	0.26	0.38	0.18	0.10
Δ3 − Δ2	%	0.26	0.38	0.52	0.52	0.44	0.40
Δ2 + Δ3	%	0.74	0.82	0.88	0.88	0.96	1.00
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	9.2	9.36	9.44	9.39	9.37	9.1
at 1.31 μm
Zero-dispersion	nm	1317.9	1314.9	1314.2	1322.6	1309.5	1308.1
wavelength
Zero-dispersion slope	ps/nm2-km	0.0906	0.0905	0.0902	0.0914	0.0897	0.0898
20 mm diameter	dB/m	0.39	0.69	1.42	1.35	0.92	0.15
bending loss
at 1.31 μm
SBS threshold	dBm	12.0	12.8	13.0	13.7	12.3	12.0
at 1.55 μm 20 km

	TABLE 12
	Unit	Example 6g	Example 6h	Example 6i	Example 6j	Example 6k	Example 6l	Example 6m
R1	μm	1.56	1.51	1.68	1.58	1.81	1.35	1.41
R2	μm	3.13	3.33	3.69	4.10	3.62	2.97	2.82
R3	μm	4.76	4.91	5.43	5.74	5.51	4.37	4.28
R2/R1	—	2.0	2.2	2.2	2.6	2.0	2.2	2.0
Δ1	%	0.56	0.56	0.50	0.50	0.44	0.50	0.44
Δ2	%	0.24	0.22	0.15	0.18	0.18	0.35	0.20
Δ3	%	0.70	0.70	0.70	0.55	0.55	0.80	0.70
Δ1 − Δ2	%	0.32	0.34	0.35	0.32	0.26	0.15	0.24
Δ3 − Δ2	%	0.46	0.48	0.55	0.37	0.37	0.45	0.50
Δ3 + Δ2	%	0.94	0.92	0.85	0.73	0.73	1.15	0.90
Fiber cutoff	μm	1.32	1.32	1.32	1.32	1.32	1.32	1.32
wavelength
Cable cutoff	μm	1.24	1.24	1.24	1.24	1.24	1.24	1.24
wavelength
MFD(Petermann II)	μm	8.54	8.83	9.64	10.03	9.77	8.1	9.2
at 1.31 μm
Zero-dispersion	nm	1322.4	1321.7	1317.1	1315.6	1310.9	1317	1314
wavelength
Zero-dispersion slope	ps/nm2-km	0.088	0.090	0.093	0.093	0.092	0.087	0.090
20 mm diameter	dB/m	0.01	0.05	0.40	1.36	0.64	0.00	0.18
bending loss
at 1.31 μm
SBS threshold	dBm	11.7	12.6	13.7	14.3	13.1	10.8	12.6
at 1.55 μm 20 km

Example 7

FIG. 22 shows a refractive index profile of an optical fiber preform of Example 7. The optical fiber preform in this example, as shown in FIG. 22, is constituted by: a three-layer structured core which includes, a first core disposed in a region from the center of the core to a radius of R1, a second core which surrounds and comes in contact with the first core and is disposed in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and is disposed in a region from the radius of R2 μm to a radius of R3 μm. Like Examples 1, 2, 5, and 6, the three-layer structured core is included. However, unlike in Examples 1, 2, 5, and 6, the refractive index profile smoothly changes, and the definition of the boundary thereof is the same as in Examples 3 and 4.

Structural parameters of the optical fiber preform of Example 7 are represented in Table 13, and optical characteristics that were exhibit when the preform was drawn into an optical fiber are shown. The optical fiber drawn from the optical fiber preform of Example 7 had an SBS threshold of 12.6 dBm for a length of 20 km and obtained a suppression effect higher than the SMF having the same MFD by +3.8 dB, and the G652 standard was satisfied.

	TABLE 13
	Unit	Example 7
	R1	μm	1.41
	R2	μm	3.03
	R3	μm	4.60
	R2/R1	—	2.1
	Δ1	%	0.49
	Δ2	%	0.13
	Δ3	%	0.64
	Δ1 − Δ2	%	0.36
	Δ3 − Δ2	%	0.51
	Δ3 + Δ2	%	0.76
	Fiber cutoff wavelength	μm	1.32
	Cable cutoff wavelength	μm	1.24
	MFD(Petermann II)	μm	9.39
	at 1.31 μm
	Zero-dispersion	nm	1319.6
	wavelength
	Zero-dispersion slope	ps/nm2-km	0.090
	20 mm diameter	dB/m	0.21
	bending loss
	at 1.31 μm
	SBS threshold	dBm	12.6
	at 1.55 μm 20 km

INDUSTRIAL APPLICABILITY

According to the invention, for the optical fiber having a segment-core type refractive index profile, the relationships between the relative refractive index differences Δ1, Δ2, and Δ3 of the layers are suitably designed, and the position of the third core is suitably determined. Therefore, it is possible to increase an SBS threshold by +3 dB or higher as compared with the SMF having the same MFD while maintaining the optical characteristics described in G652.

In addition, by allowing the relative refractive index difference of the third core to be greater than that of the first core, it is possible to improve manufacturability of the optical fiber preform.

Claims

1. An optical fiber having a refractive index profile constituted by:

a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and

a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index,

wherein

Δ2 is equal to or less than 0.4%,

Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1,

when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%,

Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15,

a cable cutoff wavelength is less than 1260 nm,

a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm,

a zero-dispersion wavelength is in the range of 1300 to 1324 nm,

a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km),

a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and

an sbs threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

2. An optical fiber having a refractive index profile constituted by:

a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and

a cladding which surrounds and comes in contact with the tee-layer structured core and has a substantially uniform refractive index,

wherein

Δ2 is equal to or less than 0.4%,

Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1,

when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied,

X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%,

Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15,

a cable cutoff wavelength is less than 1260 nm,

a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm,

a zero-dispersion wavelength is in the range of 1300 to 1324 nm,

a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km),

a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and

an sbs threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

3. An optical fiber preform having a refractive index profile constituted by:

a three-layer structured core which includes, in the central portion of the core, a first core having a substantially uniform positive relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a substantially uniform positive relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a substantially uniform positive relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and

a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index,

wherein Δ2 is equal to or less than 0.4%,

Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1,

when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%,

Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, and

when the optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an sbs threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.

4. An optical fiber preform having a refractive index profile constituted by:

a three-layer structured core which includes, in the central portion of the core, a first core having a maximum relative refractive index difference Δ1 in a region from the center of the core to a radius of R1 μm, a second core which surrounds and comes in contact with the first core and has a minimum relative refractive index difference Δ2 in a region from the radius of R1 μm to a radius of R2 μm, and a third core which surrounds and comes in contact with the second core and has a maximum relative refractive index difference Δ3 in a region from the radius of R2 μm to a radius of R3 μm; and

a cladding which surrounds and comes in contact with the three-layer structured core and has a substantially uniform refractive index,

wherein Δ2 is equal to or less than 0.4%,

Δ1, Δ2, and Δ3 have relationships of Δ1>Δ2, Δ3>Δ2, and Δ3>Δ1,

when Δ1, Δ2, and Δ3 have relationships of Δ1−Δ2=X and Δ3−Δ2=Y, (X+Y)>0.4% is satisfied, and X and Y satisfy 0.25%<X<0.6%, 0.1%≦Y≦0.6%, and a relationship of (2*X−0.7)%<Y<(X/2+0.4)%,

Δ2, Δ3, R1, and R2 satisfy relationships of (Δ2+Δ3)+1.0≦R2/R1≦7*(Δ2+Δ3)−1.45 and Δ2+Δ3≦1.15, and

when the optical fiber preform is drawn into an optical fiber, a cable cutoff wavelength is less than 1260 nm, a mode field diameter is in the range of 7.9 to 10.2 μm at a wavelength of 1.31 μm, a zero-dispersion wavelength is in the range of 1300 to 1324 nm, a zero-dispersion slope is equal to or less than 0.093 ps/(nm²·km), a uniform bending loss is equal to or less than 2 dB/m at a diameter of 20 mm and a wavelength of 1.31 μm, and an sbs threshold at a wavelength of 1.55 μm is higher than that of a single-mode optical fiber having a typical step-index profile and the same mode field diameter by +3 dB or higher.